The transmission of water or moisture through fabric or clothing assemblies is an important property—significant to the thermal comfort of clothing.
There are numerous standard test methods for determining both moisture transmission and liquid water transport through fabrics or clothing. The most commonly used methods for evaluating the moisture transmission rate or moisture vapour resistance through fabrics or clothing assemblies are the upright cup method (ASTM E96-80-Procedure B) and the Sweating Hotplate Method (ISO 11092). The former determines the water loss from a dish covered with a fabric sample over a specific period of time. Test results are normally reported in g/m2/24 hrs. The latter method determines evaporative heat loss over a gradient of water vapour pressure and reports the results as water vapour resistance in m2 Pa/W.
There are four general test method types for measuring the wicking or water transport properties through fabrics—namely: longitudinal wicking “strip” tests; transverse (or Transplanar) wicking “plate” tests; aerial wicking “spot” tests; and Syphon tests [1].
Two longitudinal wicking “strip” tests are established as industrial standards—namely: BS3424 Method 21 (1973)—Determination of Resistance to Wicking; and DIN 53924 (1978)—Determination of the Rate of Absorption of Water by Textile Materials (Height of Rise Method). Both of these methods use a preconditioned strip of test fabric suspended vertically with its lower end immersed in a reservoir of distilled water to which a die of a type known not to affect the wicking behaviour may be added for visually tracking the movement of the water. After a fixed period of time, the height reached by the water in the fabric above the water level in the reservoir is measured. The BS3424 Method 21 specifies a very long time period (24 hours) and is intended for coated fabrics with very slow wicking, whereas DIN 53924 specifies a much shorter time for the test (5 minutes maximum) and is appropriate for relatively rapid-wicking fabrics. Since water transport in clothing is generally transplanar (across the fabric plane), the measurement of a fabric's wickability in the fabric plane as measured by longitudinal wicking “strip” tests has limited implications to the measurement of clothing comfort.
Insofar as transverse (or transplanar) wicking “plate” tests are concerned, no published standards exist to date. An apparatus for this test was first implemented by Barus et al [2] and consisted of a horizontal sintered glass plate fed from below with water from a horizontal capillary tube, the level of which is set so that the upper surface of the plate is kept damp, as a simulation of a sweating skin surface. A disc of test fabric is placed on the plate and held in contact therewith under a defined pressure applied by placing weights upon it. The position of the meniscus along the capillary tube is recorded at various time intervals as water is wicked through the fabric layer. Given the diameter of the capillary tube, the recorded position of the meniscus can be used to calculate the mass transfer rate of water into the fabric. A disadvantage of this method is that the resistance to flow imposed by the capillary tube decreases and the hydrostatic head decreases during the course of the test as water wicks up through the fabric sample.
Hussain and Tremblay-Lutter [3] also described a test for measuring the transplanar uptake of liquids by fibrous materials is in contact with a liquid. An instrument adopting a DAMT (Dynamic Absorbency Measurement Technique) had a liquid reservoir which continuously supplied liquid to sample cells via pipes. The changing weight of the reservoir was measured by an electronic balance to measure the rate of liquid uptake by the textile material in the sample. A disadvantage in this technique is that the control of the liquid level is dependent on the sensitivity and accuracy of the elastic properties of the mechanical spring which is supporting the liquid reservoir.
There are two published aerial wicking “spot” test standards of measurement, namely: BS3554 (1970), Determination of Wettability of Textile Fabrics; and AATCC Method 39-1977—Evaluation of Wettability. In these standard tests, a drop of liquid (either distilled water, or for highly wettable fabrics—a 50% sugar solution) is delivered from a height of approximately 6 millimetres onto a horizontal fabric test specimen. The elapsed time between the drop reaching the fabric surface and the disappearance of a reflection from the liquid surface is taken as a measure of how quickly the liquid has spread over and wetted the fabric surface. The disadvantage of this method is that the supply water is not continuous as is the case during sweating.
Another aerial wicking “spot” test method is known as Moisture Management of Textiles (MMT) [4]. Moisture management indexes are determined by MMT for a textile sandwiched between two electrically conductive plates by measuring changes in electrical resistance across the plates. These measurements indicate changes in water content. A quantity of liquid is poured upon the upper surface of the fabric. The liquid is then transported three-dimensionally through the fabric. Based on the measurement of electrical resistance between the upper and lower plates via the surfaces of the fabric, moisture management indexes (which included accumulated liquid absorption of the upper and lower surfaces of the fabric; maximum difference between the water content of the upper and lower surfaces; and initial absorption speeds and drying rates at the upper and lower surfaces) can be calculated.
Tanner [2] and Lennox-Kerr [5] report syphon tests. A rectangular strip of test fabric is used as a syphon with one end immersed in a reservoir of water or saline solution and the other end placed at a lower level in a collection beaker. The amount of liquid transferred during successive time intervals can be determined by weighing the collection beaker. This is a simple test, but does not simulate the transport of liquid through clothing during sweating.
In each of the methods described above, there are two fundamental disadvantages. Firstly, the water level in a reservoir (cup, beaker, or tube etc) reduces as water vapour is transmitted through the fabric or as liquid water is absorbed and transported through the fabric. The reduction in water level results in an increase in the air-exposed surface area of the sample and a change in water pressure between the fabric and the water surface, which results in inconsistent test results. Secondly, in many of the tests described, it is not possible to measure water loss (transmitted in vapour form or transported in the form of water) continuously in real-time.